Photodynamic therapy (PDT), which is the interaction of light with hematoporphrin derivative (HDP, Photofrin II), is being studied for possible clinical use in several areas. Currently, PDT is being investigated in the following: cancer of the prostate (beagles); carcinoma in situ of the bladder and urinary tract (foxhounds); and pulmonary metastatic disease (nude mice). Specially designed fiber optic probes and introducers were manufactured for all cases, and calibrated in terms of irradiance delivered to the tissue. Photosensitised skin is a side effect of PDT, and the possibility of using agents to protect against this photosensitivity was addressed. In particular, hydrochlorothorazide appears to have a pronounced effect on reducing skin photosensitivity in nude mice models. Dosimetry for PDT is complex due to the variable irradiation levels within the tissue, sequestering of HDP, and tissue oxygenation. To address the first of these points and to permit light distribution within tissue to be formulated, measurements were made of light emanating from tissue as a function of radial distance from the point of injection of the light into the tissue. Theoretical analyses permit extraction of two parameters, the mean free path length between isotropic scattering events and the absorption coefficient, with and without the presence of HDP, for each of the tissues studied. These include muscle, bladder wall, liver, kidney, skin, etc. For these tissues, flux levels as a function of distance from the point of light injection have been established for three different wavelengths, 514nm, 660nm, and 780nm.